Due to the increasing need for low voltage (LV) DC supply for high power applications such as electric vehicle charge station, oil and gas fields, and other electrification needs, studies on medium voltage (MV) AC to low voltage DC converter are required. The main problem in developing these types of converter is lack of commercial availability for efficient medium voltage switches. While Si-based switches cannot generally meet the efficiency and high frequency switching requirements, SiC-based switches are not available above the 1700 V at scale. Therefore, a step down transformer is typically required to reduce the voltage for a DC-DC conversion. An Input Series Output Parallel (ISOP) multilevel structure with an additional DC/DC stage allows using low voltage semiconductor switches at medium voltage level, as well as reducing the EMI and harmonics. Major research needs to develop this topology are development of a high power high frequency transformer and advanced controls to ensure high efficiency.  

This proposed converter research project will develop a novel architecture for medium voltage AC to low voltage DC converter offering several improvements over the existing state-of-the-art technologies including compact size, modular structure, scalability to different voltage and power level, and high efficiency. The proposed converter structure is selected as series input parallel output multilevel converter with Dual Active Bridge (DAB). The design will be performed for a 1 MW AC-DC-DC converter. It will have superior advantages such as galvanic isolation with high frequency transformer that will accept 13.8 kVAC medium voltage from the grid and generate direct 1 kVDC to supply loads. To reach 13.8 kVAC voltage level with commercial available SiC switches, 11 modular AC-DC-DC converter structures are required for each phase. Every module power level is planned for 30 kW to a total of 1 MW. Each module has two main stages: AC-DC rectification and DC-DC converter stage. In the first stage, when AC-DC power conversion is performed in unity power factor, a robust control structure is proposed for voltage balancing operation between the modules. In second stage, DAB converter and High Frequency (HF) transformer will be investigated. Although, DAB converters are widely used in power conversion especially Solid State Transformer structure, they have some drawbacks such as circulating current at low power/duty cycle level and high peak current value problems as a result excessive losses at transformer and switches. The proposed algorithm suggests a new RMS current reduction control to reduce losses and to increases the power density.  On the other hand, the high frequency transformer design is essential for DAB converter. For this purpose, core material, wire selection and winding placement will be investigated. Due to time and budget constraint, a three level single-phase prototype will be developed instead of 11 levels. 277 VAC input voltage will be converted to 400 VDC voltage level. Each module power will be designed as 3 kW. This will provide the necessary knowledge, expertise, and experience to ultimately develop the three-phase MV converter.